It does not refer to "mayday" the international distress signal used in aviation and maritime radio communications to indicate a life threatening emergency.
It is aka International Workers' Day, a public holiday in many countries in the name of those struggles and achievements of the labor movement. In the United States and Canada, a similar observance called Labor Day, occurs on the first Monday of September.
It's all a bunch of commie instigated malarkey.
So this European festival is of ancient origins and hearalds the beginning of Summer. Traditions include the gathering of wild flowers and green branches and the weaving of floral garlands. The maypole is a central feature of the festivities. It is part and parcel of the ancient spring rituals from which May Day has arisen as observed in various cultures across Europe and beyond to us here in North America.
Regional variants include Walpurgis Night in central and northern Europe, the Gaelic festival called Beltane, the Welsh festival known as Calan Mai, and May devotions to the Blessed Virgin Mary. It is also associated with the ancient Roman festival Floralia.
The aforementioned commie instigated malarkey called International Workers' Day observed on 1 May is also called "May Day", but the two have different histories and much different meanings to the participants. One is a leftist libtard contrivance. The other a bonafide holiday.